There have been previously known apparatus and methods for controlling and interconnecting mass storage devices such as disk drives in a large computer system. Digital Equipment Corporation, Maynard, Mass., sells a subsystem known as the MAS bus for controlling mass storage devices used with its VAX series of computers. In like manner, Sequent Computer Systems, Beaverton, Oreg., has used storage module device disk drives (SMD) and controllers in its Balance 8000 and S-Series computers. Such disk controller systems and disk drives are capable of high performance and large data capacity, but the cost, performance, and reliability of such systems has needed improvement. Such systems also have depended on a few large storage capacity disk drives for data storage. Whenever such a drive failed, the entire computing system usually failed with it, and large amounts of valuable data were potentially lost.
Disk drives and controllers for personal computers have been developed that utilize the SCSI bus standard for control and transfer of data to be stored. The SCSI standard is described in the Draft Proposed American National Standard document entitled, "Small Computer System Interface-2 (SCSI-II)," as prepared by the Accredited Standards Committee of the Computer and Business Equipment Manufacturers Association.
FIG. 1 illustrates the major components of a typical mass storage system 10. A SCSI bus 12 interconnects a host adapter 14 with controllers 16A, 16B, and 16C (collectively "controllers 16"). Host adapter 14 and controllers 16 are functionally similar and are commercially available circuits such as model numbers 53C90 or 5380 available from NCR Corporation of Colorado Springs, Colo. SCSI bus 12 is terminated at each end by termination networks 18A and 18B (collectively "terminations 18") of types described in the SCSI standard document. SCSI bus 12 includes a termination power conductor 20 that is coupled to a termination power supply 22 through a diode 24 and a fuse 26. Host adapter 14 is in communication with a computer 28, and controllers 16 are in communication with peripheral devices 30A, 30B, and 30C (collectively "peripheral devices 30"). In mass storage system 10, peripheral devices 30, usually disk drives, and controllers 16 are commonly embedded in the same peripheral device housing and powered by embedded power supplies operating from switched AC-line power sources.
The large installed base of personal computer installations has driven the development of faster and larger capacity SCSI bus-based disk drives at decreasing costs. The performance/cost ratio of SCSI bus-based mass storage systems has improved so much that large system manufacturers, such as Sequent Computer Systems, now prefer to use SCSI bus-based mass storage systems with its computers.
SCSI bus-based mass storage systems typically use a large number of disk drives to achieve the required data capacities. There are benefits to distributing data across a large number of smaller capacity drives including faster average access time, improved mass storage system reliability, and reduced data loss in the event of a drive failure.
A problem with SCSI-I bus-based mass storage systems is that only eight device addresses are possible, thereby limiting such systems to one host adapter 14 and seven controllers 16. If more than seven peripheral devices 30 are required, multiple host adapters 14 must be added to mass storage system 10.
Another problem with SCSI bus-based mass storage systems is that the SCSI standard allows the use of non-keyed bus connectors. Such connectors can be installed backwards, thereby causing short-circuit conditions for all the bus driver circuits and the termination power. When they occur in the field, such conditions cause fuse 26 to blow out, thereby resulting in a costly field repair.
A third problem is the serviceability of systems having many peripheral devices 30. Equipment cabinets designed to contain multiple peripheral devices 30 must be designed to enable removal and replacement of individual devices while the remaining devices are still operating. The removal of individual devices also tends to adversely affect the cooling and power stability of the remaining devices. Designing and managing the cabling and configurations of such systems is also difficult.
The SCSI-II standard has addressed some of the problems inherent in the SCSI-I standard. The device addressing has been increased to a limit of sixteen devices, and higher performance cable and connector schemes have been incorporated into the SCSI-II standard. However, the connector keying, cooling, power distribution, and serviceability problems remain. There is also a need to maintain SCSI-I compatibility for reasons of field upgradability and compatibility with existing systems.